The present invention relates to the art of blowmolding freestanding plastic drums, and more specifically to an improved method for forming bungholes in blowmolded drums.
Plastic drums are often manufactured by extruding a tubular parison into a chamber defined by open mold sections, inserting a blow pin into at least one end of the parison, closing the mold sections which compresses a portion of the parison against the blow pin to form the bunghole, blowing air through the blow pin and into the parison so as to expand the remainder of the parison into contact with the interior surface of the closed mold, opening the mold, and then removing the final blown article from the mold and the blow pin.
In such a method, the blow pin generally has an annular shoulder which forms a sealing surface on the molded bunghole for abutting a reciprocal surface on the bung or closure. Additionally, the blow pin assembly generally would include means for providing the bunghole with threads for cooperative engagement with a closure. Such means might consist of threads on the blow pin itself for casting the image onto the molding plastic, or it might consist of a threaded plastic or metal insert temporarily affixed to the blow pin but over which the plastic may be molded and which remains as a part of the bunghole upon removal of the blow pin.
One of the problems association with the sealing surfaces of bungholes made by the above method is that the surfaces are somewhat conical, generally sloping upwardly and outwardly from the center axis of the bunghole. This requires the use of a highly elastic and compressible gasket to effect a seal.
One of the shortcomings of the compression molded plastic threads has been their poor definition leading to cross-threading and reduction of their mechanical advantage. Cross-threading or other thread damage commonly occurs in one of the following three manners: (1) the forced engagement of a difficult-to-screw-in closure or other device, (2) the force of impact under government-required drop tests (see United States Department of Transportation Regulations Governing Transportation of Hazardous Materials, Sec. 178.19, Specification 34), and (3) the gravitational leverage exerted by engaged pumping or other dispensing equipment.